FIG. 1 shows the basic general topology of a power amplifier 10. An impedance transformer 12 interfaces a load RL to an active device M and a power supply VDD. In this example the active device M is a transistor. One function of the impedance transformer 12 is to maximize the efficiency of energy transfer from the supply to the load. For this, it should ideally transform the load impedance RL to a value Rx seen at the drain of the transistor M, so that for given power delivered on RL, the drain of the active device swings fully from 0 to 2VDD.
For maximum efficiency, therefore,
                                                        V              DD              2                                      2              ⁢                                                          ⁢                              R                x                                              =                                                    V                L                2                                            2                ⁢                                                                  ⁢                                  R                  L                                                      =                                                            P                  L                                ⁢                                  ⟶                  yields                                ⁢                                  R                  x                                            =                                                V                  DD                  2                                                  2                  ⁢                                                                          ⁢                                      P                    L                                                                                      ,                            (        1        )            where VL is the voltage across the load RL, and PL is the power delivered to the load RL. In the case where the active device M is a differential amplifier, equation (1) becomes
                              R          x                =                                                            (                                  2                  ⁢                                                                          ⁢                                      V                    DD                                                  )                            2                                      2              ⁢                                                          ⁢                              P                L                                              =                                                    2                ⁢                                                                  ⁢                                  V                  DD                  2                                                            P                L                                      .                                              (        2        )            
The optimum value of Rx thus depends on the supply voltage and the power delivered to the load. For example, while working at constant supply voltage and reducing the power delivered to the load (power step-down), high efficiency may be maintained by increasing Rx accordingly. Alternatively, while working under variable supply voltage and delivering constant power to the load, varying the value of Rx allows for the most efficient operation.
There may also be applications with requirements for operating the power amplifier at its maximum achievable efficiency while varying both the supply voltage as well as the power delivered to the load. Such examples include battery-attached power amplifiers, power amplifiers that are required to meet the specifications of multiple standards etc.
In most modern realizations of integrated power amplifiers, the impedance transformer is implemented by means of a planar integrated transformer comprising primary and secondary inductors having inductances L1 and L2 respectively. The load RL is coupled across the secondary inductor. Such a transformer transforms the load impedance RL by a ratio n according to:
                              n          =                                    1              k                        ⁢                                                            L                  2                                                  L                  1                                                                    ,                            (        3        )            where k is the magnetic coupling factor linking the primary and secondary inductors, and 0≦k≦1. The transformation ratio is thus defined by the values of the primary and secondary inductances, as well as the magnetic coupling factor. These values, however, are set by the physical geometry of the transformer and are difficult or impossible to change once the design of the chip is in place. This limits the maximum efficiency achieved by the power amplifier when operating under different supply voltages and/or at different power levels.
One solution to this problem has been to provide one or more switches to switch in or out sections of the transformer, thus altering the inductances and the transformation ratio n as the power supply or load requirements vary. However, the switches themselves require power to operate and thus result in a loss of efficiency.